Generator interconnect - 5kV-15kV 5MW

[david h]

I've done quite a few generator plants that tie into medium voltage utility networks. They ranged in size from 1.5MW to 5MW. The utility side in our area is 15kV grounded WYE. I've always specified the generators with a grounded WYE at either 5kV or 15kV but with a with a neutral grounding resistor. We use a transformer with a solidly grounded WYE for the utility connection and a DELTA on the generator side. This allows the neutral grounding resistor to work and gives the utility company the solid WYE that they require.

We've got a new project in the works and the utility company engineer is asking for a solidly grounded WYE-WYE transformer. If we hook it up with the generator neutral bonded to the transformer, we lose fault limiting of the neutral grounded resistor. Is there some other way to do this that I'm missing?

Many thanks

 

[waross]

I was always under the impression that the delta facing the generator distributed unbalanced loads much better on the generator.
A line to neutral load on the wye side becomes a line to line load on the delta side.
In the event of a lost phase the remaining load will be better shared by the generator windings.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

There is another issue that the utility engineer may be concerned with.
A grounded wye/delta will back-feed into an open phase.
In the event of a downed conductor, both ends will be hot.
Your transformer bank will supply 1/3 of its Available Short Circuit Current plus DC offset into a fault on the utility side.
Consider differential protection on the transformer windings that simultaneously trips the main breaker, the excitation and the prime mover.
Removing the delta winding will remove the backfeed and fault level concerns.
You may be able to negotiate permission to resistance ground the generator side of the wye/wye transformer or to float the wye point and depend on the generator resistance ground.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

The delta-wye is a better solution for your side of the system, so I'd push for that. The delta will act as a grounding source for the utility distribution system, so that could be one of their concerns. If there is a breaker on the utility side, it shouldn't be too difficult to use a directional ground OC relay to trip off for utility side ground fault. With the wye-wye, the ground fault current will be limited by the generator NGR.

I'd ask the utility to explain why they feel they need (or want) the wye-wye connection. In my part of the world, utility transformer service typical commercial or industrial systems (not generators) are invariably wye-wye. So maybe the utility engineer is just assuming this should be the same.

Cheers,

Dave

 

[Hoxton]

An important question (you may already know the answer) is where is the Connection Point? This is where your responsibility ends and the Utility takes over responsibility?

Is there a clear document detailing what the utility expects from each type of connection? Especially important for both sides where there is generation on the customer’s side. If there is, what does it say about your type of connection?

Your connection is at 15 kV, do you have only generation, or are there consumer loads to be supplied? If so then you should consider having the earth connection at the incoming transformer. Then if the generator is ever removed (it may need to be taken away for repair!) there is still an earth connection. The generator can be operated unearthed, as long as it is only operated in parallel with the earth from the Utilities transformer present.

The Utility engineer has to understand that the generator will make a fault contribution, that is why we usually fit a neutral earth resistor to limit the contribution. If this resistor is not fitted to the generator, then it has to go in the transformer neutral.

 

[prc]

I have seen some transformer engineers (John Winder) commenting that delta on LV side of GSU transformers is to avoid zero sequence current entering the generator windings during a LG fault on HV side of transformer. Generator windings cannot stand zero sequence currents and will be subjected to damaging vibrations.
Dunki-Jacobs,grounding expert,in his book(chapter 9) mentioned that small generators (diesel driven) can be solidly grounded and connected to grounded star system, provided zero sequence reactance of generator is more than positive sequence reactance. If not generator neutral should be grounded through a reactor. He is recommending this as otherwise LG fault current will be more than 3L fault current in generator windings.
Can generator experts guide us?

 

[david h]

Thanks for the responses. There aren't any loads being supplied at 5kV. The only published direction from the utility is to have a solidly grounded WYE on their side, 15kV but a new guy wants the WYE WYE interposing transformer.

I've only ever used an interposing transformer on arrangements like this with a WYE Utility side and DELTA customer side. The reason is very simple, we can resistor ground the generator neutral to limit fault current. I'm pressing to go with WYE - DELTA. I just can't see any way to use a WYE-WYE and still keep the resistor ground.

 

[waross]

I see this twp ways.
As a generator operator I want my generators to see a delta winding, especially if the load is unbalanced.
As a system operator I don't want to see any wye/delta transformers on my distribution circuits.
I spent about 15 years in a land where wye/delta transformers were common on residential distribution circuits.
I made a lot of money helping customers mitigate the effects of the wye/delta transformers.
Note that residential circuits means mostly residential.
It was rare to find a residential circuit without at least one commercial or small industrial customer with a wye/delta supply.
One chronic effect is a high number of refrigerator and freezer burn-outs following a power outage or the loss of a phase.
Then I became the system engineer for a small island system serving about 5000 clients.
It took several years to get the two major wye/delta banks switched over to wye/wye.
I then banned the connection of any wye/delta banks on the system.
I did continue with wye/delta transformers facing the generators.
But it depends.
It depends on the size of the distribution circuit relative to the generators.
It depends on the switching and sectionalizing method.
Are the distribution circuits isolated with three phase switching with breakers or switchers versus fused cutouts.
Are there customers on the 15 kV circuit.
How common are phase loss events?
How well balanced are the phase voltages?
Any unbalance in the phase voltages on the 15 kV line will cause heavy circulating current in your wye/delta transformer.
I expect that the utility has a clause in their standard agreement allowing them to ban any devices on the customer side which cause unwanted effects on the grid.
If the engineer uses that clause to ban a wye/delta transformer you have no argument.
Suggestions: Subject to input from a few others, you may consider a deta/delta transformer.
You may be able to negotiate permission for a solid connection from the generator neutral to the transformer neutral and then a common resistance connection to ground.
That will give some protection to the generator but will not help with the issues that prc mentioned.
Consider an instantaneous trip off line in the event of heavy zero sequence currents.
Consider a high resistance connection from the generator neutral to the transformer neutral.


Bill
--------------------
"Why not the best?"
Jimmy Carter